1. Field of the Invention
The present invention relates to a method for producing a semiconductor crystal and to an apparatus for crystal production. In particular, the method relates to a production method useful for producing a Group 13 element nitride crystal, and to an apparatus for crystal production. The present invention also relates to a Group 13 element nitride semiconductor crystal.
2. Description of the Related Art
An ammonothermal method is a method for producing a desired material using a nitrogen-containing solvent such as an ammonia solvent in a supercritical state and/or a subcritical state and utilizing the dissolution-precipitation reaction of the starting material therein. In the method, when applied to crystal growth, a supersaturation state is generated through the temperature difference based on the temperature dependence of the solubility of the starting material in the solvent such as an ammonia solvent, thereby precipitating a crystal. In a hydrothermal method similar to the ammonothermal method, water in a supercritical and/or subcritical state is used as the solvent for crystal growth, and the method is applied mainly to a crystal of oxide such as quartz (SiO2), zinc oxide (ZnO) or the like. On the other hand, the ammonothermal method is applicable to a nitride crystal, and is utilized for growth of a crystal of nitride such as gallium nitride or the like.
The gallium nitride crystal growth according to the ammonothermal method using, for example, an ammonia solvent as the solvent for the method is a reaction in a supercritical ammonia environment at high temperature and high pressure (500° C. or higher, 150 MPa or more); and it is not easy to plan apparatus resistant to the environment and to select materials resistant thereto. The solubility of gallium nitride in pure ammonia in a supercritical state is extremely low, and therefore for increasing the solubility and promoting the crystal growth, a mineralizer is added to the system. The mineralizer is classified into an acid mineralizer such as typically ammonium halide NH4X (X═Cl, Br, I) and a basic mineralizer such as typically an alkali amide XNH2 (X═Li, Na, K). The supercritical ammonia environment containing such a mineralizer is an extremely severe corrosive environment. A pressure container (autoclave) can be produced using a material having a strength resistant to the temperature and the pressure (for example, Ni-based superalloy such as Alloy 625, RENE 41, etc.), which, however, could not have complete anticorrosive performance resistant to supercritical ammonia. In particular, the acid mineralizer is highly corrosive to the above-mentioned alloy, therefore requiring establishment of anticorrosive technology using a material having high anticorrosive performance. In case where such an acid mineralizer is used regarding this, a noble metal (platinum, iridium, platinum-iridium alloy), of which the anticorrosiveness has been confirmed, is used as a material for inner lining of autoclaves or as a material for cylindrical reactors (Patent References 1, 2).